The invention relates to an electronic trip device associated with a circuit breaker for protecting an alternating current net system, comprising:
a current sensor for measuring the current intensity flowing in each line R, S, T of the net system, PA1 a time-lag tripping circuit for controlling the signal representative of the current delivered by the sensor, and comprising a non-linear function generator which approximates the inverse time delay tripping characteristic of the circuit breaker, PA1 a level detector delivering a control order to a switching member when the output signal of the time-lag tripping circuit exceeds a predetermined threshold in reply to an excessive increase of the line current, PA1 and a tripping coil, the excitation of which is driven by the switching member to cause the opening of the circuit breaker contacts.
Some non-linear function generators permit an ideal tripping characteristic I.sup.2 t=k to be obtained, but still require a complex electronic circuit comprising a circuit to square the signal representative of the monitored current, and an integrating additional circuit connected to a level detector. An operational amplifier is necessary for each function, and the complexity of this connection is particularly expensive for low ratings circuit breakers.